Re: {Collins} KWM-2A tuning drag

You're right on, Don!
Recently I fixed a KWM-2 that had taken a hard knock to the front: hard enough to break the tuning knob. I completely disassembled the tuning mechanism and PTO and applied all the usual fixes: the main shaft and consequently the dial and mask were still was a tiny bit eccentric, and slipped and dragged badly at 2 spots in its range. Loosened the idler gear screw just enough to move the little tube (idler gear bearing) back a mm or so. Fixed! The main shaft still has a slight eccentricity, but you don't see or feel it in the tuning! I'll keep that in mind now, whenever I get itno KWM or S-line tuning. A too tightly meshed idler gear can definitely lead to drag, too, even if everything is perfectly concentric. 

73  Fred VE1FA

At 02:57 PM 29/06/2011, you wrote:
Wow Don, that's a good spin. I tried my various smooth (and what I thought were easy) PTO's on KWM-2A's and other S-line and all I get is about 20-40 Kc spin. It would be helpful to hear others experience as well if only to give people an idea of what a really nicely adjusted dial mechanism can achieve. 

73 Jerry
K1JOS

-----Original Message-----
From: vanwagnd@xxxxxxx [mailto:vanwagnd@xxxxxxx]
Sent: Wednesday, June 29, 2011 11:59 AM
To: whitebear1122@xxxxxxxxxxx; Bill Turini
Cc: Collins net
Subject: Re: {Collins} KWM-2A question
In addition to all the previous comments that help reduce PTO shaft drag and dial slippage, an additional cause of slippage and a "tight feel" is the idler roller that engages the teeth on both dials. There can be a point at which the dials push too tight against the roller increasing friction and the pinch roller slips. This often occurs at the same two points if the dials are not perfectly concentric on the mounting hub. Easiest solution is to loosen the screw that mounts the idler wheel on the front panel and move the wheel outward perhaps 1/64" away from the dials. I set the idler wheel as far away from the dials as I can and still engage the dial teeth. I have a weighted tuning knob on my KWM-2A and with all adjustments in good order I can spin the dial from 100 to 170 on the dial. 
73, Don K4LAJ


---- whitebear1122@xxxxxxxxxxx wrote:
In my own experience with my KWM-2 and S-line , it's most likely due to dried out grease. While none of my PTO's actually 'stuck', they were not smooth, had a drag to them. When I opened the 75s-3 PTO, there was dried out grease all along the shaft, which is what caused the drag. I cleaned off all the grease and relubed it with Lubriplate. Tuning is now smooth.
I've never seen a Collins PTO "spin" like you mentioned. I've got 5 Collins radios with those PTO's and even all cleaned up and with weighted knobs they simply do not spin. There is no flywheel to keep the momentum going probably too much drag from the screw mechanism with the travelling ferrite core. So I wouldn't expect any S-line/KWM PTO's to "spin" like you see on a National NC-300 where that dial may go from end to end on a good spin.
No I don't think you're nitpicking at all about the 'tight' or drag. It's annoying when it isn't smooth.
Now something I have very little experience with is that eliptical bushing behind the tuning knob. That can affect the drag of the PTO as well. When I relubed the 75S-3 PTO, it took some doing to get that bushing setting right otherwise the drag was higher and it would pinch on one side of the rotation. Maybe some of the experienced eliptical bushing guys can weight in. I'd like to hear more discussion about aligning the PTO housing on the chassis and adjustment of the bushing. 



73, Scott WA9WFA

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